A Brief History

Since early times, man has been fascinated with discovering the origins of the cosmos. Similarly, man has often been influenced by his creationist ideas: that some divine power created the universe and everything in it. For example, the Ancient Greeks developed some of the earliest recorded theories of the origin of the universe. Unfortunately, many of these Greek philosophers and astronomers placed the Earth in the center of their models of the universe. They thought, if the heavens are divine, and the gods created man, well then certainly the universe must be geocentric, meaning the Earth is the center of the universe.

Ancient societies were obsessed with the idea that God must have placed humans at the center of the cosmos (a way of referring to the universe). An astronomer named Eudoxus created the first model of a geocentric universe around 380 B.C. Eudoxus designed his model of the universe as a series of cosmic spheres containing the stars, the sun, and the moon all built around the Earth at its center. Unfortunately, as the Greeks continued to explore the motion of the sun, the moon, and the other planets, it became increasingly apparent that their geocentric models could not accurately nor easily predict the motion of the other planets.
Take the apparent motion of Mars from an observer on the Earth, for example. As the Earth and Mars orbit around the sun, Mars appears to advance forwards, and then stop and start moving backwards, and then stop and change direction once again to start moving forwards (shown in the picture at left). You can see in the picture that this phenomenon is easily explained by a heliocentric universe ("heliocentric" meaning the sun is the center of the universe), but imagine being an ancient Greek and trying to understand why Mars would follow such an unusual orbit (when, according to them, it was supposed to have a circular orbit) if the Earth was the center of the universe!
After Aristotle developed a more intricate geocentric model (which was later refined by Ptolemy), general cosmology clung to these misconstrued ideas for the next 2,000 years. Even when Nicholas Copernicus introduced the notion of a heliocentric universe, many contemporary societies greatly influenced by religious beliefs refused to accept it. Today we consider this a ridiculous question; we can directly observe that the Earth and the other planets in our solar system orbit around the sun. It is obvious that the technological advancements of the 20th century have allowed us to “look out” to the farthest corners of the “visible universe”, but our past history of erroneous assumptions should make us cautious. As we discover more and more about the origins of our early universe, we should realize that our present theories must be continually tested and modified because new theories frequently arise as we learn more through our observations. That is why most physicists and astronomers today are so inclined to accept the Big Bang Theory as the most plausible explanation for the origin of the universe. It puts together so many of the pieces of how the universe came into being, and seems to correct so many of the flaws found in previous theories.

Until the 1920s, cosmology was still dominated by the theory of a Steady State Universe, or the idea that the universe was homogeneous (has the same general make-up throughout), infinite (that the universe just extends forever), and static (the universe is not expanding, it just is). If you just study the night sky, it seems easy just to think that this is the way the universe has always looked and will always look. In the 20th century, however, observations of the universe did not seem to add up with Steady State theory…

Return to top.

Home | Introduction | Brief History | Olbers' Paradox | Hubble Expansion | Big Bang Theory | Electromagnetic Spectrum | CMB | Glossary of Terms